Revert of Changed scavenge GC to collect unmodified references (patchset #9 id:160001 of https://codereview.chromium.org/1358703003/ )

Reason for revert:
[Sheriff] Speculative revert due to crbug.com/539814

Original issue's description:
> Changed scavenge GC to collect unmodified references
>
> Added a scavenge GC pass that collects unmodified references instead of
> processing object groups.  This mode can be controlled by setting
> FLAG_scavenge_remove_unmodified_objects. By default this is turned off.
> Also, modified a test case to suit the handle the new GC pass.
>
> BUG=v8:4421
> LOG=N
>
> Committed: https://crrev.com/6254019238a853c9f3c09d615ba153043f6957c7
> Cr-Commit-Position: refs/heads/master@{#31102}

TBR=jochen@chromium.org,rmcilroy@chromium.org,mythria@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4421,chromium:539814

Review URL: https://codereview.chromium.org/1388133002

Cr-Commit-Position: refs/heads/master@{#31130}